1. Field of the Invention
The present invention relates to the field of radio antennas, and in particular to a no-loss, multi-band antenna structure.
2. Background of the Invention
The use of directional antennas for radio communications is well-known in the prior art. Of particular relational interest to this invention is the Yagi-Uda directional design. This type of antenna consists of two or more antenna elements arranged either horizontally or vertically, with all elements in the same plane and parallel to each other. A typical Yagi-Uda antenna is shown in FIG. 1, in which there are four (4) total elements, with the second element 1 from the "Reflector" end 2 (the far right) being the fed (active) element, or "Driver". The elements forward (to the left) of the driver are known as directors, reference numerals 3 and 4 (i.e., Director 1 and 2). The elements not driven directly are known as parasitic elements. This type of antenna can be tuned in various ways to accomplish specific design goals, such as forward gain, front-to-back ratio and operating VSWR (voltage standing wave ratio). Elements can be added, or deleted in the same process. For further information on Yagi-Uda antennas, see, for example, J. L. Lawson, Yagi Antenna Design (American Radio Relay League 1986). Multiples of these antennas can also be fed simultaneously in a variety of phase relationships between the individual antennas. The operating frequency of a Yagi-Uda antenna is limited to a few percent of the center design frequency. To enable wider frequency coverage, two methods have been classically employed.
One common method is the use of traps placed in specific locations of the elements. One such antenna is shown in FIG. 2 and is known as a "tri-band" antenna, since it can operate in portions of three distinctive frequency bands (14.0-14.35,21.0-21.45 and 28.0-29.7 MHz). This type of antenna can be a single element (single dipole), or parasitic elements added in a Yagi-Uda style design. Another common method is the log periodic antenna, in which every element is active, that is, driven directly and not parasitic in nature. It is shown in FIG. 3. This type of antenna can operate over a wide frequency range, limited by the number of elements utilized and the boom length. A range of several megahertz (MHz), such as 14-30 MHz is not uncommon. A hybrid of the log periodic and Yagi-Uda designs is the so-called "log-yag", in which there is a log periodic type driven cell and one or more parasitic elements, as in FIG. 4. Parasitic elements can also be located in various locations within a classic log periodic antenna to augment particular frequency segments within the normal operating range of the log periodic antenna. FIG. 5 is a hybrid multi-band antenna utilizing a two-element log driven cell as well as trapped and non-trapped elements to cover five (5) frequency bands.
The prior art designs described above operate on multiple, as opposed to single, frequency bands only by making substantial sacrifices in performance. Although the multi-element trapped antenna can use fewer elements to cover more frequency ranges, the elements are not spaced optimally, they are not optimally tuned and there are losses associated with the traps. A single trapped dipole also has losses in the traps. Consequently, a Yagi-Uda design with trapped elements represents a compromise design for gain, front-to-back ratio and overall efficiency.
The log-periodic antenna compromises forward gain for wide operating bandwidth. It is believed to exhibit about the same forward gain as a moderately tuned three element Yagi-Uda of short boom length in terms of wavelength, or about 6 dBd. Compared to the usual 3-4% operating bandwidth of a Yagi-Uda, this type can be 25% of the center design frequency.
Another method of realizing wider frequency coverage with an acceptable operating VSWR is the open-sleeve antenna. This is lesser known, although it was invented by Dr. J. T. Bollijahn, of Stanford Research Institute, in about 1946. The typical implementation is shown in FIG. 6, which is the first page from a comprehensive article on the open-sleeve antenna. The antenna consists of three (3) elements: a center element and two "sleeve" elements of equal length and tuned to a higher frequency than the center element, usually at half the frequency. The primary purpose of the open-sleeve antenna is to obtain wider VSWR bandwidth and also operate on two frequencies with a single feedline, as stated in the article.
A commercial utilization of the open-sleeve is contained back in FIG. 2. Here, the open-sleeve is combined with traps to make a three-band driven cell. The center element contains traps and operates on 14 and 21 MHz, whereas the sleeve elements are tuned to 28 MHz. The common driver is the trapped element and the system is fed with a single feedline. The wider bandwidth is achieved when the frequencies of the central and sleeve elements are closely related.